The present invention relates to a system and method for controlling the recording of data on a strip chart to avoid recording undesired data.
Strip chart recorders find use, inter alia, as trend recorders for physiologic data, such as heart rate. Due to the simultaneous detection of the desired physiologic signal and interfering signals, monitoring equipment will at times produce unreliable data. Signal processing techniques often enable the detection and elimination of such unreliable data.
However, when the data is being recorded on a strip chart as a trend record, it is often necessary to cease recording when data fails to be a reliable indicator of bodily function. Various techniques have been adopted for this purpose. For example, an active mechanical device can be added to the recorder to lift the pen from the strip chart. Such devices add substantial expense and complexity to the recorder.
Another technique utilizes thermal pens with short time constants, so that the heating current is turned off and no recording made while the data is unreliable. However, it is necessary to maintain the heated surface of the pen as small as possible to achieve a sufficiently short thermal time constant, and in most recorders, the pen sweeps in an arc over a knife edge to print on the strip chart paper drawn over the edge. It is, therefore, necessary to add an expensive articulating mechanism to convert this arcuate motion to a linear motion of the pen over the knife edge to maintain the thermally active section of the pen on the strip chart paper. Moreover, should the pen become misaligned with the knife edge, no recording would be made.
Yet another technique in use simply drives the pen off the scale of the strip chart at the limited slew rate at which data is recorded and holds it there until reliable data is once again available. This produces a succession of lines on the paper which are distracting and render it difficult to determine the significance of the reliable data which has been recorded.